Using a 3-pin led to trigger a relay

Thread Starter

tournesol123

Joined Nov 25, 2023
2
Hello,
I am using a DC-DC Boost Step-up Voltage Converter featuring a 3-pin LED charge indicator to charge a battery.
The LED is red while charging the battery. When the charge is completed (out current is 0 amp), the LED turns green.
I need to connect a relay to the LED and activate it when the LED is green.
When I connect the related 2 pins of the LED to an optocoupler, it activates it but the LED turns OFF, apparently because the optocoupler steals its current.
How can this be resolved, keeping in mind that the green LED shares its cathod with the red one within the 3-pin LED?
Thanks in advance for your hints and suggestions.
 

Ian0

Joined Aug 7, 2020
9,499
A circuit diagram would help, but, at a guess, you could put the opto-isolator LED in series with the green LED (provided that the circuit driving it has enough voltage to drive them both)
 

LesJones

Joined Jan 8, 2017
4,172
The red LED will be fed via a resistor from a higher voltage to limit the current. If the red LED on the board is connected in parallel with the IR LED in the opto coupler the IR LED will clamp the voltage to about 1.2 volts. (LEDs behave a bit like a zener diode) So both LEDs will have 1.2 volts across them. The red LED requires about 1.8 volts to light up. You would need to connect the LED in the opto coupler via it's own current limiting resistor to the point where the limitinf resistor for the red LED is connected. It is possible that the current limitig resistor for the red LED is omitted and they just use the internal current limiting of the device that drives it.

Les.
 

Thread Starter

tournesol123

Joined Nov 25, 2023
2
Hello,
I am using a DC-DC Boost Step-up Voltage Converter featuring a 3-pin LED charge indicator to charge a battery.
The LED is red while charging the battery. When the charge is completed (out current is 0 amp), the LED turns green.
I need to connect a relay to the LED and activate it when the LED is green.
When I connect the related 2 pins of the LED to an optocoupler, it activates it but the LED turns OFF, apparently because the optocoupler steals its current.
How can this be resolved, keeping in mind that the green LED shares its cathod with the red one within the 3-pin LED?
Thanks in advance for your hints and suggestions.
Thank you for your feedback!

For some reason, I couldn't make it work with either an optocoupler or a miniature solid state relay. At the end, things worked out using a 2N2222 transistor. Since the LED was shutting off, I added a resistor to reduce the transistor's consumption, which dimmed the LED by an acceptable level but kept it lit.

The project is finally working now : when the LED turns red, the converter's cooling fan turns on. When the battery gets charged, the LED turns green and the fan turns off, prolonging its life and my ears' as well:)

Tadoom-Fan-Relay-2N2222.jpg
 
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